Metal oxide silicon field effect transistors (MOSFETs) are highly susceptible to damage from exposure to electrostatic discharges. The gate conductor of a MOSFET device is separated from an underlying source, drain and conduction channel semiconductor region by a very thin insulating layer. The insulating layer is typically constructed of silicon dioxide (SiO.sub.2) having a thickness of about 200 angstroms. The breakdown voltage of a high quality silicon dioxide layer of such thickness may only be about 20 volts. Electrostatic voltages may range from several hundred volts to several thousand volts. Such voltages can be easily generated and discharged by a person touching the terminals of an integrated circuit, or the equipment housing the circuit. Therefore, when the gate conductor of a MOSFET device is used as an input to a packaged integrated circuit, the inadvertent application of an electrostatic voltage thereto can destroy the input transistor.
One approach previously utilized in providing electrostatic discharge protection is to connect a Shockley diode (a two terminal SCR) to the gate of the input transistor. The Schottky diode is formed as a four layer device with alternate P and N junctions. The disadvantage with this approach is that when the Schottky diode is fabricated in accordance with conventional integrated circuit processing steps, such diode does not break down until the electrostatic voltage reaches about one hundred volts. It is apparent that with one hundred volts applied to the input of a MOSFET integrated circuit, it is highly likely that the circuit will be damaged. The one hundred volt breakdown of the Schottky diode arises from the formation of an N-well in a P-type substrate to fabricate one junction of the four layer diode device. This junction exhibits the largest breakdown voltage of the Schottky device, which voltage must be exceeded in order to turn on the diode.
Other attempts to provide electrostatic discharge protection include the provision of a gateless MOSFET transistor connected across the input device to be protected. Under normal operating conditions, the protection device would remain in a nonconductive state, as it has no gate or conduction channel. Rather, there is formed in lieu of a conduction channel an insulating silicon dioxide which allows conduction therearound only when a relatively high voltage is impressed between the semiconductor source and drain regions. This approach requires a substantial amount of wafer area, added input capacitance to the circuit, and is generally difficult to fabricate with a closely controlled breakdown voltage.
From the foregoing, it can be seen that a need exists for an improved method and circuit for protecting the inputs of semiconductor circuits. Particularly, a need exists for clamping electrostatic voltages to a safe level without damaging either the circuits to be protected, or the protection circuit itself.